We are still in the process of looking into the many changes we would like to make to the Graph Editor.

Nonetheless, one of the things you had asked for is already available:

Key(s) may be dragged either horizontally or vertically. Hold Shift then click on the key and start drag in the desired direction. Preferred direction is guessed from the direction of the very first mouse move.

Tangent editing may be constrained the same way. Hold Shift, click the tangent extremity and start drag either:- along the tangent direction to modify tangent length only (angle is locked), - perpendicular to the tangent direction to keep tangent length and only rotate it.

Hope this helps until we add more features to the Graph Editor, Best regards,Greg